Ion exchange treatment (defined as A+BXB+AX, where A and B are both cations or both anions) typically involves the interchange/sorption of ionic species between a liquid phase (containing the contaminant) and a suitable solid phase using batch or continuous flow processes, until the specific sites available for ion exchange/sorption in the solid phase become saturated, and in the case of the continuous process, the contaminant/toxic ions appear in the eluant. Treatment is then stopped, and the contaminant ions present on the solid phase are subsequently exchanged with other ions. In this way the contaminant ions are recovered and the exchanger is regenerated.
Inorganic sorbents are used extensively in industry and in research areas such as radiochemistry, geochemistry, and radiopharmaceuticals. Many of their industrial applications are within the broad context of waste remediation, particularly in areas such as nuclear waste decontamination, mine tailings management, and various hydrometallurgical separations.
A particular application of inorganic sorbents is as ion exchangers in the decontamination of liquid nuclear wastes, such as for example those formed during the dissolution of fuel elements using strong acids. These liquid nuclear wastes are usually stored in underground stainless steel tanks. Typically in legacy wastes, that is those wastes which have been stored for times typically exceeding about 5 years, 137Cs and 90Sr are the radionuclides responsible for most of the radioactivity (with minor contributions from 99Tc and 106Ru). Such wastes can result from weapons development and power generation activities. Being highly soluble, Cs and Sr can easily become mobilised and find their way into the biosphere representing a significant danger to public health. It is therefore highly desirable to remove such radioactive elements selectively from the nuclear waste streams, particularly at the pre-treatment stage, and thereby allow safe disposal of the bulk of the waste as low level waste. This pre-treatment option for nuclear waste management is being implemented by many governmental agencies and independent contractors responsible for waste minimisation and the safe disposal of nuclear wastes.
Accordingly, a wide range of ion exchangers, both organic and inorganic, have been tested for potential use in pre-treatment of liquid nuclear waste.
Numerous organic (polymeric) ion exchangers are known and have been utilised to treat nuclear waste streams, with a number of these being very selective for Cs and Sr. In general however, such polymeric ion exchangers have inherently low radiation stability and are consequently not suitable for interim storage. Because of this they are widely regarded as being less favourable than inorganic systems for nuclear waste remediation.
Inorganic ion-exchangers are preferred for nuclear waste applications because inorganic materials have greater radiation stability and hence maintain their efficacy longer.
However, despite the recognised advantages of using inorganic sorbents to pre-treat liquid radioactive wastes, particularly for the uptake of Cs+ and Sr2+, there remains a need for inorganic sorbents that can simultaneously sorb practical quantities of Cs+ and Sr2+ from highly acidic aqueous streams. Such acidic waste streams which are generated through dissolution of spent nuclear fuel elements in nitric acid, are particularly relevant. Further, there is a need for reusable inorganic ion exchangers for the pre-treatment of acidic waste streams, including liquid nuclear waste, to selectively remove contaminant metal ions, thereby resulting in waste having a lower level of contamination. Such low-level waste could then be effectively disposed of with an enormous cost saving compared to the expense in disposing of non pre-treated liquid nuclear waste.
Hydrous metal oxides, including those of niobium, antimony, and tungsten, are known to sorb various cations. Most of these compounds are either amorphous to X-rays and/or do not possess well defined microporosity or surface chemistry. Microporous oxides of the above named metals are, however, known to have structures containing well defined tunnels. Of particular relevance here are those with the so-called hexagonal tungsten bronze (HTB) and pyrochlore structures shown in FIG. 1. It is important to note that in the term ‘hexagonal tungsten bronze’, ‘hexagonal’ refers to the symmetry of the unit cell while the term ‘bronze’ is not a structural one, but refers to the fact that the compounds often have metallic lustre. Thus, the term ‘hexagonal tungsten bronze’ as used herein refers to a class of compounds with the HTB or HTB-like structure which do not necessarily contain tungsten.
Certain HTB compounds are well known and have been synthesised in various ways, ranging from DC magnetron sputtering, which produces materials with the formula WO3, to low temperature hydrothermal methods such as those used for zeolite preparations that yield hydrated fine particle materials with the general formula NaxWO3. ZH2O. The latter methods usually employ acidified sodium tungstate solutions which are heated to temperatures in the range 100-300° C. to effect crystallisation.
It is here disclosed for the first time that certain HTB and pyrochlore compounds, including fine particle, hydrothermally-prepared HTB and pyrochlore compounds, are capable of selectively removing certain ions from solutions containing much higher concentrations of sodium ions, including acidic solutions. In particular it is disclosed that certain HTB and pyrochlore compounds are capable of selectively removing both Cs and Sr ions simultaneously from aqueous solutions. These properties make them useful in applications involving environmental decontamination.